In semiconductor microlithography, circuit patterns from a mask or a reticle are projected onto a sensitized surface of a wafer. The mask and a wafer are aligned so that the appropriate areas of the wafer are exposed to the corresponding circuit patterns. In addition, the mask and wafer must be aligned so that patterns from multiple exposures are accurately registered.
The alignment of the mask and wafer is typically determined using a position detector that measures the relative positions of the mask and reticle using alignment marks on the wafer. Some position detectors operate according to a Field Image Alignment ("FIA") system. In an FIA system, alignment marks are illuminated with a broadband light flux from a light source such as a halogen lamp. The alignment marks are then imaged onto a light sensitive surface of an image sensor such as a CCD and the position of the alignment marks is determined by processing the image signal from the image sensor.
The alignment marks are generally patterns formed on the wafer by a patterned metal or insulating layer. These layers can be as thin as about 10 nm and can be as thick as about 1 .mu.m. The position measurements obtained with FIA alignment sensors tend to be inaccurate because the images of the alignment marks have low contrast. Images of alignment marks formed of very thin layers of an insulator have particularly poor image contrast.
FIA position detectors that have improved position accuracy, even with alignment marks made of thin layers of an insulator, have been disclosed in Japanese laid-open patent documents HEI 3-27515 and HEI 7-183186. The alignment sensor of Japanese patent document HEI 7-183186 detects the position of such alignment marks by defocusing the image of the alignment marks. Consequently, alignment errors are caused by the tilt of the chief ray with respect to the optical axis. To correct this error, the tilt of the chief ray (the deviation from telecentricity) must be measured.
The alignment sensor of Japanese patent document HEI 3-27515 uses bright-field imaging to image thick-layer alignment marks while the thin-layer alignment marks are imaged using phase-contrast imaging. In order to switch from bright-field imaging of thick layers to phase-contrast imaging of thin layers, the aperture stop of the illumination system is removed and a phase plate is inserted into the imaging optical system. As a result, it is difficult to maintain optimum imaging conditions for both bright-field and phase-contrast imaging.
In addition, because the illumination conditions are changed by removing the aperture stop, switching from bright-field to phase-contrast imaging changes the aberration balance in the image. In bright-field imaging with a large illumination numerical aperture, the illumination light flux nearly fills the entrance pupil so that image aberrations are determined primarily by the total aberrations of the pupil. However, in phase-contrast imaging the numerical aperture is smaller and the illumination light flux fills a only a small area of the entrance pupil. Image aberrations are determined only by this small area. As a result, it is difficult to maintain optimum imaging for both phase-contrast and bright-field imaging. Furthermore, the positioning of optical elements for phase-contrast imaging and bright-field imaging cannot generally be done independently so that image quality is degraded.
Some position detectors improve image contrast for thin alignment marks by providing a phase plate having an area of reduced transmittance. While such a phase plate improves image contrast, the imaged is formed with a reduced light flux intensity. The reduced flux intensity decreases the accuracy of alignment mark measurements done with conventional image sensors.